The present invention relates to filtering, and in particular to a method and a device for high-pass filtering of a signal.
As is known in the art and illustrated in FIG. 18A, a high-pass filter 200 may be used as part of a low-pass filter arrangement for low-pass filtering of a signal in(k). As such, the filter arrangement removes high-frequency signal components from the input signal in(k), possibly after amplification or attenuation by an amplifier 218, to provide a low-pass filtered signal as the output signal out(k). A delay element 215 equalizes any signal delays introduced by the high-pass filter 200.
Traditional high-pass filters are linear filters, such as the FIR filter 200 of FIG. 18A, which possesses a transfer function h(z)=0.25·[−z−2+2z−1−1]. With respect to the input signal in(k), the filter provides an output signal value of three consecutive input signal values, the output signal value corresponding to a normalized difference between the middle of the three input signal values and the mean value of the other two input signal values.
The mode of operation of the low-pass filter of FIG. 18A for the input signal in(k) illustrated in FIG. 18B is illustrated in FIG. 18C. The low-pass filter filters out high-frequency signal components, but also pulse-like signal components are decreased and widened at their base, and edges of the input signal are partly smoothed out. The result is that the filter also filters out, to an undesirable degree, the pulses and signal steps from the input signal.
The range of application of such filter arrangements is very large, and the discrete signals that are filtered are formed, for example, by sampling a signal that is continuous over time. Such signals can also be graphical information values, which are taken from an image line-by-line or column-by-column. In the processing of television signals, there are known methods in which frames with graphical information in every line are generated from the transmitted fields, in which only every other line contains graphical information. In the event of an incorrect interpolation of the intermediate-image lines, a signal variation may be produced in the vertical direction of the picture, where maximum and minimum signal values alternate from line to line (for example, luminance values or chrominance values). Such high-frequency signal processes need to be filtered, for example to generate a gray value from a black/white variation from one line to another, in every line. However, individual signal steps, represented as brightness edges or color edges in the picture, should remain unaffected by the filter process. Similarly, signal pulses, i.e., narrow bright or dark regions in the picture, should not be removed or attenuated by the filter process.
What is needed is a method and a device for the high-pass filtering of a signal, which filters out high-frequency signal components but does not filter out signal pulses or signal steps, or does so only to a small degree.